Development of microstructure during heat treatment of high carbon Cr–Mo steel

Abstract

Stainless steels containing enhanced chromium and carbon contents are particularly attractive for applications requiring improved wear and corrosion resistance. The as cast microstructure of such steels is composed mainly of ferritic matrix along with a network of interdendritic primary carbides. It has been shown that heat treatment of these steels results in microstructures that contain more than one type of carbide. A selective dissolution technique has been employed to isolate carbides from the matrix. Scanning electron microscope and X-ray diffraction studies of the as cast steels have shown that the primary carbides are essentially of M₇C₃ type, whereas in heat treated specimens both M₇C₃ (primary) and M₂₃C₆ (secondary) type carbides have been observed. The relative amounts of these carbides are found to be dependent on the heat treatment temperature. In addition, nucleation of austenite occurs above 950°C and at ～1250°C the matrix transforms entirely to austenite, which is retained completely on quenching to room temperature.     

Carbide types in an advanced microalloyed bainitic/ferritic Cr–Mo Steel – TEM observations and thermodynamic calculations

The strength and toughness of low alloyed ferritic/bainitic steels depend on their microstructure, which evolves during thermo‐mechanical treatments along the processing chain. Chromium‐molybdenum steel microstructures are complex. Therefore, only a limited number of attempts have been made to fully characterize carbide populations in such steels. In the present work, analytical transmission electron microscopy is employed to study the microstructure of a low alloyed chromium‐molybdenum steel, which features ferritic (F, mainly α‐iron and niobium‐carbides) and bainitic (B, α‐phase, dislocation, grain/subgrain boundaries, various M_xC_y carbides) regions. The crystal structure and chemical nature of more than 200 carbides are determined and their distributions in the two microstructural regions are analyzed. The present work shows how particles can be identified in an effective manner and how the microstructural findings can be interpreted on the basis of thermodynamic calculations.     

Metallographic observations during the sintering of BM2 type of high speed steels

Metallographic changes during the sintering of BM2 type of high speed steels have been investigated by scanning electron microscopy, in the range of optimum sintering to oversintering. The primary carbides observed are M₆C and a small quantity of MC; in the oversintered structure an additional carbide with eutectic morphology was seen. It is a chromium and molybdenum rich phase in BM2+ 0 to 4% cobalt (0.9 to 1.2% carbon) alloy, whereas in BM2+ 8% cobalt (0.9% carbon) the eutectic phase is MC. Under certain conditions M₃C was also detected in the post-sintered alloy.     

Microstructural evolution in bainite,martensite, and δ ferrite of low activation Cr–2W ferritic steels

Abstract

The microstructural evolution in (2–15)Cr–2W–0·1C (wt-%) firritic steels after quenching, tempering, and subsequent prolonged aging was investigated, using mainly transmission electron microscopy. The steels examined were low induced radioactivation ferritic steels for fusion reactor structures. With increasing Cr concentration, the matrix phase changed from bainite to martensite and a dual phase of martensite and δ ferrite. During tempering, homogeneous precipitation of fine W₂C rich carbides occurred in bainite and martensite, causing secondary hardening between 673 and 823 K. With increasing tempering temperature, dislocation density decreased and carbides had a tendency to precipitate preferentially along interfaces such as bainite or martensite subgrain boundaries. During aging at high temperature, carbides increased in size and carbide reaction from W₂C and M₆C to stable M₂₃C₆ occurred. No carbide formed in δ ferrite. The precipitation sequence of carbides was analogous to that in conventional Cr–Mo steels.

MST/1049     

Solubility of copper in titanium carbide

The solubility of copper in titanium carbide (TiC_x) was studied experimentally. The solubility was assessed in two different ways: (a) titanium carbide was added to a copper—titanium melt, (b) titanium carbides formed in situ, from graphite and titanium, were analysed. The stoichiometries were implicitly assessed. The solubility of copper in titanium carbide was found to decrease with increasing carbon content. This behaviour was compared to the solubility of chromium in titanium carbide, which increases in solubility with increasing carbon content. The role of vacancies in the solution is briefly discussed.

MST/3502     

Influence of microstructure and chromium content on oxidation behaviour of spin cast high speed steels

Abstract

The influence of the chromium content and of the volume fraction of primary carbides on the thermal oxidation behaviour of spin cast high speed steels and semi-high speed steels used for the production of hot mill rolls was studied at 700°C. Oxidation nucleates at the carbide–matrix interface and carbides have a higher oxidation resistance than the matrix. Moreover carbides dissolve a higher amount of chromium than the matrix. As a consequence of these effects, the oxidation rate of these steels decreases by increasing the chromium content of the matrix and by decreasing the carbide volume fraction.     

Influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steel

The influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steels was analyzed by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The microstructure of laser melted high chromium steel is composed of austenite with supersaturated carbon and alloy elements and granular interdendritic carbides of type M₂₃C₆. Secondary hardening of the laser melted layer begins at 450 °C after tempering, and the hardness reaches a peak of 672HV at 560 °C and then decreases gradually. After tempering at 560 °C, a large amount of lamellar martensite was formed in the laser melted layer with a small quantity of thin lamellar M₃C cementite due to the martensitic decomposition. The stripy carbides precipitating at the grain boundaries were determined to be complex hexagonal M₇C₃ carbides and face centered cubic M₂₃C₆ carbides. In addition, the granular M₂₃C₆ carbides and fine rod-like shaped M₇C₃ carbides coexisted within the dendrites. As a result, the combined effects of martensitic transformation, ultrafine carbide precipitations, and dislocation strengthening result in the secondary hardening of the laser melted layer when the samples were tempered at 560 °C.     

Effect of microstructure on tensile fracture of quenched and lightly tempered high carbon and low chromium steel containing undissolved spheroidal carbides

High carbon and low alloy chromium steels have been studied to determine the effect of the microstructure on tensile fracture of quenched and lightly tempered low alloy steel containing undissolved spheroidal carbides. The steels with a volume fraction of 8 and 13 vol % and containing particle sizes from 0.32 to 1.14m were investigated. In the case of steel containing 8 vol % undissolved carbides, many twinned plates were observed in the matrix martensite and microtwinning was observed in the carbide/matrix interfaces. The steel failed in a macroscopically brittle manner and the true fracture stress of the steel was independent of the carbide particle size, while the data exhibited a large scatter. In the case of steel containing 13 vol % of undissolved carbides, the matrix martensite consisted predominantly of lath martensite and a well-defined forest of dislocations was observed around the carbides. Failure of the steel occurred in the relatively early stage of plastic deformation and the true fracture stress of the steel increased with decreasing carbide particle size.     

On the microstructure and mechanical properties of high-speed steels

The microstructure of high-speed steels consists of a martensitic matrix with a dispersion of two sets of carbides. These carbides are usually known as primary and secondary carbides. The role of the primary carbides has been reported to be of no importance in strengthening the steels, due to their large size and large interparticle spacing. The present authors have studied the role of the primary carbides on the wear of high-speed steels and found them to be of no importance, and under certain conditions contributing to higher wear rates. It has been shown analytically and experimentally that in quenched and tempered high-speed steels, the precipitation of the secondary hardening carbide (cubic M₂C type) is the main reason for the improved strength and wear resistance. This shows that the secondary hardening phenomenon of high-speed steels is a direct result of the hardening caused by the precipitation of the cubic M_2C-type carbide. The present study has estimated that at peak hardness the volume fraction of secondary hardening carbides is approximately 20%. The measured strength of high-speed steels was found to be lower than the theoretically calculated strength due to non-homogeneous precipitation of the secondary hardening carbides. Areas which were observed to be free from secondary hardening carbides are real and are not artefacts. It has been shown that the strength of high-speed steel in the region of peak hardness depends primarily on the precipitation of the secondary hardening carbide and secondarily on martensitic strengthening.     

Effect of vanadium on cast carbide in high speed steels

Abstract

The effect of vanadium (0–4%) on the morphology and amount of eutectic and eutectoid carbides in high speed steels has been investigated using scanning electron microscopy and image analysis. It was found that vanadium promotes the formation of MC carbide and M₂C carbide, but inhibits the formation of M₆C carbide. In the vanadium free steels, the eutectic carbide consists solely of skeletal M₆C. For each steel composition, there is a critical vanadium content at which the skeletal eutectic changes to lamellar eutectic and the critical value decreases as the molybdenum content of steel increases. The effect of vanadium on the total amount of eutectic carbide differs in tungsten alloyed and molybdenum alloyed high speed steels. The δ eutectoid has a rodlike morphology in tungsten high speed steels; δ eutectoid is not present in Mo–W or molybdenum high speed steels. Increasing the vanadium content leads to an increase in the size of eutectic and eutectoid carbides.

MST/1264     

Precipitates change of ferritic‐martensitic 11 % chromium steel under short‐term creep

A ferritic‐martensitic (FM) 11 % chromium steel with final heat treatment was subjected to a short‐term creep test at a stress of 150 MPa and 600 °C for 1100 h in order to study the change of precipitates in the steel during the creep test. Except for Nb‐rich metall carbides (MC, M₂₃C₆) and Laves phases, Fe‐W‐Cr‐rich M₆C (based on Fe₃W₃C) carbides forming during the creep test were also identified in the crept steel by electron diffraction and x‐ray diffraction in combination with energy dispersive x‐ray analysis of extraction carbon replicas. The identified M₆C carbides have a fcc crystal structure, a metallic element composition of approximately 44Fe, 32 W, and 20Cr in atomic %, and large sizes ranging from 100 nm to 300 nm in diameter. The M₆C carbides are a dominant phase in the crept steel. M₆X precipitates are generally not easy to form during high temperature creep, even if it is a long‐term creep, in ferritic‐martensitic 9–12 % chromium steels with a final heat treatment. The present work provides the evidence for the M₆C carbides forming during short‐term creep in ferritic‐martensitic high chromium steels. The formation of the M₆C carbides was discussed.     

Secondary carbide precipitation in an 18 wt%Cr-1 wt% Mo white iron

High chromium (18%) white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The austenite is destabilized by a hightemperature heat treatment which precipitates chromium-rich secondary carbides. In the as-cast condition the eutectic M₇Ca₃ carbides are surrounded by a thin layer of martensite and in some instances an adjacent thicker layer of lath martensite. The initial secondary carbide precipitation occurs on sub-grain boundaries during cooling of the as-cast alloy. After a short time (0.25 h) at the destabilization temperature of 1273 K, cuboidal M₂₃C₆ precipitates within the austenite matrix with the cube-cube orientation relationship. After the normal period of 4 h at 1273 K, there is a mixture of M₂₃C₆ and M₇C₃ secondary carbides and the austenite is sufficiently depleted in chromium and carbon to transform substantially to martensite on cooling to room temperature.     

Quantitative microanalysis of carbide/carbide interfaces in WC–Co-base cemented carbides

Abstract

Carbide/carbide boundaries in WC–Co-base cemented carbides containing 6–20 wt-%Co were studied with two high resolution microanalytical techniques: atom probe field ion microscopy and analytical transmission electron microscopy. All boundaries studied, i.e. WC/WC boundaries and, in materials containing cubic carbides (γ-phase), WC/γ and γ/γ boundaries, were found to contain about half a monolayer of cobalt, localized to a zone of monolayer thickness. The carbide/carbide boundaries may thus be described as grain (phase) boundaries to which cobalt has segregated. The carbide skeleton model for WC–Co is thereby confirmed. In WC–Co materials which contain Cr₃C₂ as a grain growth inhibitor, chromium segregates to WC grain boundaries.

MST/354     

GCr15轴承钢热处理过程中碳化物的析出与演变行为

采用定量金相的方法研究GCr15轴承钢在球化退火、奥氏体化淬火、低温回火等不同热处理工序后其碳化物的演变行为,通过ThermoCalc软件进行数值模拟计算分析碳化物尺寸和成分对其在奥氏体化时固溶动力学的影响。结果表明:球化退火处理后形成的碳化物粒子尺寸呈多峰分布,奥氏体化和回火后的碳化物粒子尺寸分布为单峰分布,奥氏体化后碳化物中Cr含量略有增加;Cr含量高的碳化物粒子具有较大尺寸;球化退火形成的碳化物在奥氏体化时大量固溶形成了富碳奥氏体,淬火后转变为高碳马氏体并导致高硬度;奥氏体化时碳化物固溶发生Cr的配分导致碳化物中Cr含量增加;直径200nm的碳化物即使其Cr含量接近基体成分,也不能在奥氏体化热处理时完全固溶,未溶的碳化物颗粒将影响后续回火过程的碳化物析出。     

Oxidation behaviour of ledeburitic steels for hot rolls

The oxidation behaviour of two high speed steels (HSS) employed for the production of hot rolls was studied. The steels have slightly different chromium contents and volume fractions of primary carbides. Because oxidation nucleates at the matrix–carbide interfaces and propagates in the matrix without involving the carbides, the oxide scale grows less uniform. All the primary carbides have a higher Cr content than the matrix; therefore they tend to reduce the oxidation resistance. The slight differences in chromium content and in carbide volume fraction are responsible for the different oxidation resistance at 600 and 700°C, whilst at 500°C the two steels have almost the same resistance.     

Simple preparation of tungsten carbide supported on carbon for use as a catalyst support in a methanol electro-oxidation

Two types of supported tungsten carbides were prepared via the impregnation of tungsten precursors on carbon support followed by heat treatment. Depending on whether ammonium metatungstate (AMT) or tungsten chloride (WCl₆) was used as the precursor, this process resulted in samples that are referred to as either WC-A or WC-W, respectively. Both WC-A and WC-W showed tungsten subcarbide (W₂C) as the major crystalline phase, with tungsten monocarbide (WC) as a minor phase. More amount of tungsten carbide being formed when WCl₆ was used as the precursor. This increased formation has occurred because the thermodynamically favorable properties of WCl₆ caused the contact area between the tungsten precursor and the carbon support to promote formation of tungsten carbide. The prepared tungsten carbides were used as a catalyst support of the Pt catalyst in a methanol electro-oxidation. The metal dispersion and the catalytic performance were increased as follows: Pt/C<Pt/WC-A<Pt/WC-W. It is believed that the tungsten carbides supported on the carbon support improved the dispersion of Pt and the activation of water for removal of intermediate CO, which enhanced the catalytic performance during the methanol electro-oxidation.     

Diamond formation and behaviour of carbides in several 3d-transition metal-graphite systems

The formation of diamond and the behaviour of coexisting carbides with diamond in several 3d-transition metals (manganese, iron, cobalt, nickel)-graphite systems was studied under 7 GPa pressure up to 1700° C from the view-point of diamond formation. In the nickel-graphite and cobalt-graphite systems, no stoichiometric carbide was formed, but M _x C, which is thought to be the interstitial solid solution of carbon, was formed. In the iron-graphite system, on the other hand, the formation of two stoichiometric carbides, i.e. Fe₃C and Fe₇C₃, was found. In the manganese-graphite system, only one stoichiometric carbide Mn₇C₃, which is isostructural with Fe₇C₃, was formed. The diamond formation process in the presence of these transition metals is discussed in relation to the carbide formation.     

Vanadizing carbon steels by chemical vapour deposition

A series of carbon steels (0.15–1.45% C) were vanadized by chemical vapour deposition to produce a coating of vanadium carbide. The carbide is very nearly of constant composition (VC_0.84?VC_0.89 and microhardness (2630 HV 0.025) irrespective of the carbon content of the steel. The microstructure of the carbide is equiaxed and fine grained with no apparent preferred orientation. A cementite interlayer is formed under the carbide on steels containing more than about 1.4% C.     

Interaction between the TiC(TiCN)-Ni-Mo hardmetals and chromium vapours

The interaction between TiC or TiCN-based hardmetals with a Ni-Mo binder, or cermets, and chromium vapour in a vacuum was investigated over a wide temperature range acceptable for depositing wear-resistant coatings without the formation of a liquid phase in the cermets. Computer modelling in the Ti-C-Cr system showed that a direct interaction of TiC with chromium, leading to the formation of chromium carbides, is not possible because of the high thermodynamic stability of titanium carbide. It was established experimentally that as a result of the interaction between the cermets and chromium vapours, a coating characterized by a two-layer structure was deposited on the cermet surface. The coating consists of an inner layer adjacent to the substrate, which is composed of the chromium and carbon solid solution in nickel, and an outer layer composed of a mixture of (Cr, Ni)₇C₃ and (Cr, Ni)₂₃C₆. The activation energy of the deposition process is 387 kJ mol^–1 which is close to the value of the chromium heat of evaporation. The coating deposition process is supposed to be limited by the rate of the external supply of chromium from the vapour phase. The results of the investigation of the structure, composition and morphology of the coating are presented. A mechanism responsible for the interaction of the cermets with chromium vapour leading to the formation of the two-layer coating, is proposed.     

Effect of aluminum on the primary carbides of a hypereutectic high chromium cast iron

The effect of aluminum on the primary M₇C₃ carbides of a hypereutectic high chromium cast iron containing 4.0 wt% carbon and 20.0 wt% chromium was studied by means of optical microscopy, scanning electron microscope (SEM), energy dispersive X‐ray spectrometry (EDX), water quenching, and differential thermal analysis (DTA). Compared with specimen without aluminum addition, the primary carbides were all refined when different amount of aluminum was added into the melts, but the primary carbides in specimen with 0.3 wt% aluminum were the finest. With the addition of aluminum, aluminum element enriched at the boundary of primary carbides during solidification and was beneficial for the refinement of primary carbides. However, the increase of primary carbide growth time with the increase aluminum content had adverse effect on the refinement of the primary carbides. The comprehensive influence of those two factors leaded to the result that the primary carbides in specimen 1 with 0.3 wt% aluminum were the finest.